"export" only prevents IWYU from adding these headers if the headers
that export them are included, while "private" ensures that IWYU never
adds these headers.
- The defined interface for the UI is only the RPC protocol. The original
UI interface as an array of function pointers fill no function.
- On the server, all the UI:s are all RPC channels.
- ui.c is only used on the server.
- The compositor is a preprocessing step for single-grid UI:s
- on the client, ui_client and tui talk directly to each other
- we still do module separation, as ui_client.c could form the basis
of a libnvim client module later.
Items for later PR:s
- vim.ui_attach is still an unhappy child, reconsider based on plugin experience.
- the flags in ui_events.in.h are still a mess. Can be simplified now.
- UX for remote attachment needs more work.
- startup for client can be simplified further (think of the millisecs we can save)
